OVERALL SUMMARY: IMMUNOBIOLOGY AND IMMUNE THERAPY FOR MERKEL CELL CARCINOMA Our Seattle-based MCC team, together with collaborators at several institutions, has played a leading role in characterizing the immune response against this largely virus-driven often-lethal skin cancer. We have established clinical trials targeting critical immune pathways including PD-1 blockade that have now become part of the standard of care for this aggressive disease. These efforts have recently led to the inclusion of pembrolizumab (anti-PD-1) in the preeminent guidelines for cancer care in the US and to the first-ever FDA approval for a therapy for MCC, avelumab (anti-PD-L1). Although approximately half of patients derive long-term benefit from PD-1 pathway blockade, there remains great unmet need for the nearly half of patients with advanced disease who do not persistently respond to PD-1 pathway blockade. We propose a highly focused and integrated effort to advance our understanding of immunogenic and pathogen- driven cancers based on these recent major insights in MCC. This proposal seeks to advance our understanding of why patients do or do not respond to PD-1 blockade therapy, to determine relevant immune evasion mechanisms, and to identify and prioritize therapies likely to be beneficial for this disease and other immunogenic cancers. Utilizing the unique biology of Merkel cell polyomavirus (MCPyV)-induced MCC and our extensive Specimen Repository and Relational Database, we are poised to address two paradigm-shifting issues: the utility (Project 1) and importance (Project 3) of functional, antigen-specific T cell avidity in controlling cancer, and the identification of tumor-intrinsic and innate immune-evasion mechanisms (Project 2) that can be targeted to broaden the adaptive immune response in PD-1 pathway blockade refractory patients. Project 1 will identify high-avidity anti-MCPyV T cells, conduct a clinical trial to test the safety and efficacy of CD8 T cells transduced with the antigen-specific TCRs from these high-avidity T cells, and determine the mechanisms involved with response or non-response to this cutting-edge approach. The overarching goal of Project 2 is to understand the mechanisms associated with success or failure to respond to PD-1 pathway blockade. By obtaining and comparing serial pre- and post-PD-1 blockade treatment biopsies and subjecting them to sophisticated studies by a leading team of collaborators, Project 2 will uncover targetable aspects of tumor biology, T cell biology, and innate immunity that affect the response to PD-1 blockade. Project 3 will greatly expand our prior studies of the adaptive immune response to MCC to include a far more detailed analysis of virus-specific CD8 T cells that play a key role in MCC patient outcomes. We will broaden our investigation to include tumor-specific responses by B cells and CD4 T cells, as we have recently developed tools to isolate and characterize these MCPyV- specific lymphocytes. Leveraging these collective studies, this Program will provide a unique opportunity to characterize the tumor-specific immune response against cancer antigens that are shared across MCC patients and uncover mechanisms of immune evasion that will be important for cancers more broadly.